Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-03-27
2003-03-18
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S204200, C359S205100, C359S206100, C359S207110, C359S208100, C359S216100, C347S241000, C347S244000, C347S258000, C347S259000
Reexamination Certificate
active
06535317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-scanning optical system and light-scanning apparatus using it and, particularly, is suitably applicable to image-forming apparatus, for example, such as laser beam printers, digital copiers, etc. involving the electrophotographic process, constructed to reflectively deflect light emitted from light source means (a single light source or plural light sources) by a polygon mirror as an optical deflector and optically scan a region on a surface to be scanned, through an f-&thgr; lens system having the f-&thgr; characteristics to record image information thereon.
2. Related Background Art
In the light-scanning optical systems (light-scanning apparatus) such as the laser beam printers and the like heretofore, the light emitted as optically modulated according to an image signal from the light source means is periodically deflected by the optical deflector, for example, consisting of a rotary polygon mirror (polygon-mirror), and the deflected light is converged in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by the f-&thgr; lens system having the f-&thgr; characteristics, to optically scan the region on the surface of the recording medium to effect image recording thereon.
FIG. 19
is a schematic diagram to show the major part of a conventional light-scanning optical system. In the same figure a diverging beam emitted from light source means
91
is converted into a nearly parallel beam or into a converging beam by a collimator lens
92
. an aperture stop
93
shapes the beam (light amount), and the thus shaped beam is incident to a cylindrical lens
94
having its refracting power only in the sub-scanning direction. The beam entering the cylindrical lens
94
emerges in an as-incident state in the main scanning section while being converged in the sub-scanning section, thereby being focused as a substantially linear image near a deflection facet
95
a
of the optical deflector
95
consisting of a rotary polygon mirror (polygon mirror).
Then the beam reflectively deflected by the deflection facet
95
a
of the optical deflector
95
is guided through the f-&thgr; lens system
96
having the f-&thgr; characteristics onto a surface of photosensitive drum as a surface to be scanned
97
, and the optical deflector
95
is rotated in the direction of arrow A to optically scan the region on the photosensitive drum surface
97
in the direction of arrow B (the main scanning direction), thereby recording image information thereon.
In order to implement highly accurate recording of image information in the image-forming apparatus using the light-scanning optical system of this type, it is necessary to meet the following requirements: the curvature of field is well corrected across the entire surface to be scanned, so as to equalize spot sizes; and the system has such distortion (f-&thgr; characteristics) as to establish the proportional relation between angles and image heights of the beam reflectively deflected by the optical deflector.
Meanwhile, there are demands for optical systems capable of scanning at high speed because of an increase in speed and definition of laser beam printers, digital copiers, and so on. Since there are limitations to the rotational speed of a motor, which is part of the scanning means, to the number of facets of the polygon mirror, which is part of the deflecting means, and so on, there are increasing desires, particularly, for multi-beam scanning optical systems capable of scanning the surface simultaneously with a plurality of beams emitted from a plurality of light-emitting regions (light sources).
In order to equalize the spot sizes throughout the entire surface to be scanned in such multi-beam scanning optical systems, the curvature of field needs to be well corrected for, while lateral magnifications in the sub-scanning direction need to be equalized throughout all the image heights. If the lateral magnifications in the sub-scanning direction differ depending upon image heights, spot sizes in the sub-scanning direction will vary depending upon image heights.
Unless the lateral magnifications in the sub-scanning direction are equalized throughout all the image heights, there will arise another problem that when the light-emitting regions are located off the optical axis in the sub-scanning direction as in the multi-beam scanning optical systems, scanning lines will be curved and the spacing between the lines in the sub-scanning direction will vary depending upon the image heights, thus causing degradation of image quality.
The various light-scanning optical systems for solving such problems have been proposed heretofore, for example, in Japanese Patent Application Laid-Open Nos. 8-297256, 10-232347, and so on.
The light-scanning optical system described in Japanese Patent Application Laid-Open No. 8-297256 is constructed in such structure that change in the F-number in the sub-scanning direction depending upon image heights of the beams incident to the surface to be scanned, is suppressed by continuously changing curvatures in the sub-scanning section of at least two lens surfaces of a lens constituting the f-&thgr; lens system from on the axis toward off the axis in the effective part of the lens. The above application describes the example in which the beams incident to the f-&thgr; lens system are converging beams and in which the change in the F-number in the sub-scanning direction is suppressed well.
The scanning optical system described in Japanese Patent Application Laid-Open No. 10-232347 is constructed in such structure that the two lenses constituting the f-&thgr; lens system are provided with an optimum combination of respective refracting powers in the sub-scanning section. The application describes the example in which the beams incident to the f-&thgr; lens system are converging beams and in which the spot sizes in the sub-scanning direction are equalized on the surface to be scanned.
An object of the present invention is to nullify occurrence of jitter and effectively correct an asymmetric component in the curve of scanning line occurring in use of relatively large polygon diameters and a total inclination component in the magnifications in the sub-scanning direction across the entire scanning area in a light-scanning optical system (or a multi-beam light-scanning optical system) or in light-scanning apparatus (or multi-beam light-scanning apparatus) wherein the beam incident to the f-&thgr; lens system is a nearly parallel beam, and thereby provide a light-scanning optical system capable of providing an image at high speed and with high quality while equalizing the spot sizes throughout the entire surface to be scanned and nullifying the curve of scanning line, and also provide light-scanning apparatus using it.
SUMMARY OF THE INVENTION
A light-scanning optical system according to one aspect of the invention is a light-scanning optical system comprising a first optical system for converting a beam emitted from light source means into a nearly parallel beam, a second optical system for focusing the converted beam into a linear beam along a main scanning direction on a deflection facet of deflecting means, and a third optical system for focusing the nearly parallel beam deflected by the deflecting means, on a surface to be scanned,
the light-scanning optical system being constructed so that an optical axis of an incidence optical system including the first optical system and the second optical system is inclined relative to a normal to the surface to be scanned, at least in the main scanning section,
wherein the third optical system comprises at least one optical element and the at least one optical element is arranged so that in the main scanning section a symmetry axis in the main scanning direction of the optical element is inclined relative to the normal to the surface to be scanned, so as to bring an end of the optical element on the light source means away from the deflecting means.
In one aspect of the
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Phan James
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